In the manufacture of electric motors, the larger the quantity of winding wire that can be incorporated in the core of the stator, the better the performance of the motor. In addition, motor manufacturers also seek to increase manufacturing yield. Thus, in known manner, coils are inserted into stators automatically.
The wires used in such electric motors are typically constituted by an electrical conductor coated in one or more insulating layers based on enamel.
These wires are produced at very high speeds, and they are therefore subjected to high levels of mechanical stress and friction which can damage their insulation and can lead to irregular winding. Inserting coils also leads to the same type of stresses and friction.
To solve that problem, document U.S. Pat. No. 4,693,936 discloses making the outside insulation of said winding wires by means of an enamel obtained by reacting an acid anhydride, a di-isocyanate, and a multi-functional organosiloxane.
The advantage of that kind of enamel lies in the fact that it is made up of a polymer modified by means of polysiloxane. Because polysiloxanes have greater affinity with air than with organic media, they aggregate and migrate towards the surface while the enamel is polymerizing. This migration creates a fine layer that is rich in polysiloxane at the surface of the layer of enamel, with this polysiloxane-rich layer providing the desired sliding properties.
The method described in document U.S. Pat. No. 4,693,936 for manufacturing the above-mentioned enamel consists in:                causing an anhydride to react with an isocyanate; and        causing the product obtained in that way to react with a functionalized polysiloxane.        
That method is not satisfactory. The polysiloxane does not react completely; as a result the portion of the polysiloxane that has not reacted is free within the polymer and it sublimes while the layer of enamel is polymerizing. This gives rise in particular to damage to the catalyst in the polymerizing oven, and causes it to degrade early.
In addition, the resulting enamel is not stable over time, and after a few hours the enamel is observed to segregate, with a solid deposit forming on the surface of the liquid.
Document US 2001/0020081 discloses a method of manufacturing a polyimide enamel from a functionalized polysiloxane obtained by causing 2,3,3′,4′-biphenyltetra-carboxylic anhydride (the functionalized monomer involved in synthesizing the enamel) to react with a functionalized polydimethylsiloxane.
The enamel obtained in that way is also unstable, i.e. it precipitates. In addition, it does not possess a coefficient of friction that is as low as desired.